1. Field of the Invention
The present invention relates to an oxide thin film for a bolometer. More specially, it relates to an oxide thin film for bolometer-type uncooled infrared detector having high sensitivity.
2. Description of the Related Art
Bolometer, as a kind of infrared detector, can detect infrared through measuring changes in electrical resistance caused by the temperature increase, which is occurred by absorbing the infrared radiated from an object and converting it to a thermal energy. Accordingly, an infrared image can be realized by making a 2-dimensional focal plane array of the bolometer having such characteristics.
The characteristics required for a bolometer material are a high TCR (temperature coefficient of resistance), a low device resistance, a compatibility with a IC process, a low cost and simplification in manufacturing process, a high reliability and so on.
The bolometer materials recently used are metal thin films such as Ti, a vanadium oxide, an amorphous silicon, etc. The use of metal films causes a merit of low resistance at room temperature and a demerit of a very low TCR, which requires an improvement of device responsibility. In case of amorphous silicon, the property of responsibility is pretty good because of a high TCR value, but a Johnson noise increases with an increased device resistance.
And vanadium oxide has relatively higher TCR values around −2.0%/° C. than metal film and relatively lower device resistance below 100 kΩ than amorphous silicon. Besides, vanadium oxide has problems that it is difficult to fabricate thin films with high reproducibility due to the existence of various metastable phases, accordingly the preparation of vanadium oxide films requires expensive apparatuses such ion beam and laser equipment, or high process temperature over 400° C.
In order to solve the problems of high resistance of vanadium oxide for application to bolometer device, U.S. Pat. No. 5,288,380 discloses a method for reducing the resistance of vanadium oxide by doping other metals such as tungsten, chrome, manganese into vanadium oxide.
Although this prior art is invented to reduce the resistance of vanadium oxide by doping, it just very roughly gives a description on doping effect without providing specific doping composition etc., and does not reveal an improvement of TCR value and the performance of a infrared detector caused by doping. And this prior art suggests co-sputtering in an oxygen atmosphere as a method for doping other metals such as tungsten into a vanadium oxide. Since the characteristics of a vanadium oxide are strongly dependent on an oxygen partial pressure due to existence of various metastable phases, an oxygen partial pressure has to be precisely controlled in such method for reactively producing a vanadium oxide in an oxygen atmosphere. Accordingly it is very difficult to produce a reproducible vanadium oxide thin film and control the characteristics of the vanadium oxide film.
U.S. Pat. No. 6,489,613 (Dec. 3, 2002) discloses a method for producing an oxide thin film having a high TCR and a relative low resistance value through substituting a vanadium of vanadium oxide with other metal ion (Cr, Al, Fe, Mn, Nb, Ta, Ti) by sol-gel method and a heat treatment at the temperature of 400° C. to 450° C. However, this technique has the drawbacks in viewpoint of process that the heat treatment temperature is relatively high over 400° C., and a long-term heat treatment is required including successive anneals in oxidation and reduction atmospheres after deposition of oxide thin film. Furthermore, if an oxide thin film is deposited by a sputtering method, not a sol-gel technique, the problem in reproducibility can be easily occurred due to the difficult of controlling oxygen content.